Comparison of electrochemical current noise signals arising from symmetrical and asymmetrical electrodes made of Al alloys at different pH values using statistical and wavelet analysis. Part I: Neutral and acidic solutions

• Asymmetrical (As) electrodes of Al alloys are favorable for ECN measurements in neutral solutions.
• In neutral solution, As electrodes eliminate any need to compare the partial and original signals for recognizing true scale.
• In highly acidic media, As electrodes produce less apparent unidirectional signals.
• This is mainly due to the change of cathodic reaction from ORR to HER, thereby the less hindered cathodic reaction.
• AA6061 unlike AA2024 reveals the essential characteristic of asymmetrical electrodes in acidic media.

In this paper, the electrochemical current noise (ECN) signals were measured on large symmetrical (100–100 mm2), small symmetrical (1–1 mm2) and asymmetrical electrodes (1–100 mm2) made of AA6061 and AA2024 alloys in 1.0 M chloride solutions at different pH values (6, 2, 1 and 0.4). The results were analyzed by statistical and wavelet methods. In the near-neutral solution the asymmetrical electrodes made of Al alloys were more favorable for ECN measurements compared to both the large and the small symmetrical electrodes. The asymmetrical electrodes in near-neutral solutions allowed recognizing the true timescale of the predominant transients directly from the maximum peak of the SDPS plots, so that the comparison of the partial and original signals was not necessary. According to the statistical analysis, the asymmetrical electrodes in acidic solutions gradually lost their essential characteristic for recording the unidirectional ECN signals with decreasing pH. This can be attributed to both the dissolution of the passive oxide layer and the cathodic noise arising from the hydrogen bubble evolution. According to the wavelet analysis, for the asymmetrical electrodes made of AA6061 it is possible to detect the true timescale of the predominant transients on the basis of the maximum peak in the SDPS plots, even in highly acidic solutions, while for those made of AA2024 it is necessary to compare the partial and original signals, whether at pH 2 or lower.